The phosphorylation process plays an important role in the structural organization of the neuronal cytoskeleton. Synthesis, transport and assembly of neurofilament (NF) proteins are regulated by specific kinases that extensively phosphorylate different motifs. The phosphorylation of NF proteins is developmentally and spatially regulated. Most of this phosphorylation takes place in the Lys-Ser- Pro (KSP) repeats in the carboxyl-terminal tail domain of NF-M and NF-H. This phosphorylation is believed to stabilize the NF network in the axon, and to affect the axonal transport and conduction velocity in the neurons. Some KSP sites of this tail domain in NF-H have been suggested to be phosphorylated by cyclin dependent kinase-5 (Cdk5). Cdk5 also phosphorylates tau protein and this phosphorylation occurs exclusively at the same sites found in the tau protein from the Alzheimer's disease brain. Abnormal NF phosphorylation has also been associated with neurodegenerative process in vivo, we have begun to generate gene knockout mouse models for specific kinases and their activator subunits. Cdk5 null mouse exhibits specific kinases and their activator subunits. Cdk5 null mouse exhibits unique lesions in the central nervous system associated with perinatal mortality. The brains of Cdk5 null mice lack cortical laminar structure and cerebellar foliation. In addition, the large neurons in the brain stem and in the spinal cord show chromatolytic changes with accumulation of NF immunoreactivity. These findings indicate that Cdk5 is an important molecule for brain development and neuronal differentiation and also suggest that Cdk5 may play critical roles in cytoskeleton structure and organization.